Re: [DNSOP] Question regarding RFC 8499

Robert Edmonds <edmonds@mycre.ws> Thu, 23 July 2020 17:24 UTC

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Date: Thu, 23 Jul 2020 13:24:49 -0400
From: Robert Edmonds <edmonds@mycre.ws>
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Subject: Re: [DNSOP] Question regarding RFC 8499
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Michael De Roover wrote:
> Regarding the primary and secondary servers, it's a fair euphemism but this
> among further fracturing of nomenclature in other projects makes this
> definition very fragmented (master/slave is now primary/secondary, main,
> parent/child, etc). This is something I find unnecessary and harmful, as it
> creates confusion while merely redefining the same.

"Primary" and "secondary" are not euphemisms or later re-definitions.
They appear extensively throughout STD 13 and appear to be the preferred
nomenclature, e.g. in the below description of zone transfers from RFC
1034. "Slave" does not appear anywhere in STD 13 to the best of my
knowledge; the closest reference is to "non-master" servers.

Mockapetris                                                    [Page 27]

RFC 1034             Domain Concepts and Facilities        November 1987


4.3.5. Zone maintenance and transfers

Part of the job of a zone administrator is to maintain the zones at all
of the name servers which are authoritative for the zone.  When the
inevitable changes are made, they must be distributed to all of the name
servers.  While this distribution can be accomplished using FTP or some
other ad hoc procedure, the preferred method is the zone transfer part
of the DNS protocol.

The general model of automatic zone transfer or refreshing is that one
of the name servers is the master or primary for the zone.  Changes are
coordinated at the primary, typically by editing a master file for the
zone.  After editing, the administrator signals the master server to
load the new zone.  The other non-master or secondary servers for the
zone periodically check for changes (at a selectable interval) and
obtain new zone copies when changes have been made.

To detect changes, secondaries just check the SERIAL field of the SOA
for the zone.  In addition to whatever other changes are made, the
SERIAL field in the SOA of the zone is always advanced whenever any
change is made to the zone.  The advancing can be a simple increment, or
could be based on the write date and time of the master file, etc.  The
purpose is to make it possible to determine which of two copies of a
zone is more recent by comparing serial numbers.  Serial number advances
and comparisons use sequence space arithmetic, so there is a theoretic
limit on how fast a zone can be updated, basically that old copies must
die out before the serial number covers half of its 32 bit range.  In
practice, the only concern is that the compare operation deals properly
with comparisons around the boundary between the most positive and most
negative 32 bit numbers.

The periodic polling of the secondary servers is controlled by
parameters in the SOA RR for the zone, which set the minimum acceptable
polling intervals.  The parameters are called REFRESH, RETRY, and
EXPIRE.  Whenever a new zone is loaded in a secondary, the secondary
waits REFRESH seconds before checking with the primary for a new serial.
If this check cannot be completed, new checks are started every RETRY
seconds.  The check is a simple query to the primary for the SOA RR of
the zone.  If the serial field in the secondary's zone copy is equal to
the serial returned by the primary, then no changes have occurred, and
the REFRESH interval wait is restarted.  If the secondary finds it
impossible to perform a serial check for the EXPIRE interval, it must
assume that its copy of the zone is obsolete an discard it.

When the poll shows that the zone has changed, then the secondary server
must request a zone transfer via an AXFR request for the zone.  The AXFR
may cause an error, such as refused, but normally is answered by a
sequence of response messages.  The first and last messages must contain



Mockapetris                                                    [Page 28]

RFC 1034             Domain Concepts and Facilities        November 1987


the data for the top authoritative node of the zone.  Intermediate
messages carry all of the other RRs from the zone, including both
authoritative and non-authoritative RRs.  The stream of messages allows
the secondary to construct a copy of the zone.  Because accuracy is
essential, TCP or some other reliable protocol must be used for AXFR
requests.

Each secondary server is required to perform the following operations
against the master, but may also optionally perform these operations
against other secondary servers.  This strategy can improve the transfer
process when the primary is unavailable due to host downtime or network
problems, or when a secondary server has better network access to an
"intermediate" secondary than to the primary.

-- 
Robert Edmonds